Direct and indirect effects of a shoot-base boring weevil and plant competition on the performance of creeping thistle, Cirsium arvense

Creeping thistle or Canada thistle, Cirsium arvense (L.) Scop., is consider ed one of the world's worst weeds and the third most important weed in Euro pe. Biological control of this indigenous weed in Europe by use of native a gents may provide a low-cost alternative to use of chemical or mechanical c ontrol measures and contribute to a more sustainable weed management. We in vestigated the potential of a shoot-base boring weevil, Apion onopordi Kirb y (Coleoptera: Apionidae), for biological weed control, in the presence or absence of plant competition by three grass species. Infestation of thistle shoots by A onopordi at natural infestation levels reduced above- and belo wground plant performance after 2 years. Plant competition at natural level s had an overall greater effect than that of herbivory, significantly reduc ing both above- and belowground thistle performance in both years, thereby slowing the propagation of the weed. Weevil infestation and grass competiti on had a synergistic effect on C. arvense growth; the combined effects of t he two factors was greater than the sum of both single-factor effects. The experiment revealed that A. onopordi promotes systemic infections of the ru st fungus Puccinia punctiformis (Str.) Rohl in the year following weevil in festation. Systemically infected thistle shoots died before the end of the growing season. Although the direct effect of A. onopordi may not be suffic ient to control creeping thistle, the synergistic interaction with plant co mpetition and the indirect effect via promotion of systemic rust infections makes A. onopordi a promising agent for the biological control of this wee d. (C) 2001 Academic Press.